When a potential higher than a basic potential of unit battery (cell) is necessary, such as a hybrid vehicle using a lithium ion cell as a power source, it is common to use a plurality of unit batteries which are connected in series. However, even though the batteries are produced with the same structure via a typical production method using the same anode, cathode and electrolyte material, a difference in charging or discharging (and self discharging) characteristics exists between each of the batteries connected in series.
Therefore, a potential difference can exist between the unit batteries when using the batteries connected in series. Even if one battery of the unit batteries connected in series is perfectly discharged regardless of a potential of another battery, the battery is required to be recharged. Upon recharging the battery, since the potentials of the batteries are different from one another, there is a problem of over-charging in that the battery may reach a prescribed voltage in advance, as well as a problem of charge inefficiency in that some batteries may not reach the prescribed voltage even in an existence of an over-charging.
Further, when the number of charging/discharging times becomes large, degradation of materials included in the battery occurs to vary a property of the battery, and as a result, such degradation situation is responsible for further increasing a difference in individual cells.
In order to address such problems, there are proposed various charge-equalization apparatuses which can accomplish charge equalization of the batteries connected in series.
As an example, Korean Patent Publication No. 2008-0080864 is directed to a system that detects at least one first cell requiring cell balancing according to a plurality of cell voltages measured in a sensing unit for measuring each cell voltage of many cells, performs cell balancing on at least one first cell based on another method according to operation modes of a vehicle, and share a single cell balancing driving unit by a switch module in cell balancing. Korean Patent Publication No. 2006-0078967 is directed to a system for preventing an error by temperature and deterioration of a voltage detecting unit and detecting exact cell voltage, the system including: a plurality of battery cells; a plurality of cell relays connected to each of the cells; a voltage detecting unit including an amplifier; a sensing unit for measuring a temperature; and a microcontroller unit (MCU). The system is characterized in that the single voltage detecting unit is shared by individual cells through the cell relay.
Korean Patent Publication No. 2003-0096978 includes a plurality of unit cells, a charging means, a discharging means and a serial-to-parallel converting switch. The related art is directed to a system for performing charging by connecting the discharged unit cells in series by using the serial-to-parallel converting switch after equally discharging each unit cell. Korean Patent Publication No. 2007-0064244 is directed to a system, including a cell unit, a field effect transistor connected to the cell unit, an amplifying unit connected to the field effect transistor, a multiplexing unit for controlling an output signal of the amplifying unit, a comparing unit for comparing and determining variation of a voltage signal of the cell unit, an analog-to-digital (A/D) converting unit for converting the output of the comparing unit into a digital signal, a micro computer unit for receiving the signal output in the A/D converting unit and outputting a signal corresponding to charging/discharging conditions, a switching unit for supplying a battery balance current by operating according to signals of the micro computer unit, and a known charging/discharging circuit.
JP Patent Laid-Open No. 2008-196869 relates to a system having a feature that a capacitor is connected to each cell of an assembly cell including a plurality of cells in parallel, and the voltage of each capacitor is sequentially connected to a voltage detecting device by using one switching unit in a capacitor and the other switching unit in a ground. JP Patent Laid-Open No. 2005-0284297 relates to a system, including an assembly cell including a plurality of cells, a sampling switch for sampling voltage of each cell, a capacitor for detecting voltage of each cell, a transfer switch for transferring charge voltage charged to the capacitor, a ground potential setting switch for connecting a reference potential to a ground terminal of a voltage detecting device to sense voltage charged in the capacitor, and a voltage detection control means for controlling each switch and receiving voltage of the capacitor. JP Patent Laid-Open No. 1998-032936 is related to a system including a plurality of unit cells, a means for detecting remaining capacity of each unit cell, a charge and discharge replacing means for performing charging and discharging of each unit cell, a controller for individually controlling charging and discharging of each unit cell, and a direct current-to-direct current (DC/DC) converter performing charging and discharging of each unit cell independently. JP Patent Laid-Open No. 2004-194410 is related to a system including at least two unit cell groups, a means for detecting a difference between currents flowing each of a first cell group, and a second cell group, a means for controlling charging/discharging current of the cell group based on the current difference.
However, in the related charge equalizing apparatuses, since each of batteries connected in series is equipped with a charge equalizing apparatus or a voltage sensing apparatus to perform charging or discharging of individual batteries, there are limitations in that complexity and volume of the charge equalizing apparatus are increased thereby decreasing productivity and increasing production costs, and devices for controlling each component and forming a current route should withstand a high voltage stress.